1. Field of the Invention
This invention relates generally to an elevated temperature forming method and preheater apparatus for fabrication of complex deep drawn panels such as door inners, lift gates, deck lids and hoods from sheet metal workpieces comprising a metal, such as aluminum or magnesium, having insufficient formability at lower temperatures.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Elevated temperature forming and the preheating of sheet metal workpieces is known in the art. For example, U.S. Pat. No. 6,463,779 issued 15 Oct. 2002 to Terziakin, discloses a preheating system that includes placing a sheet metal workpiece on a press table or lower die of a die set and rapidly preheating the workpiece to a desired temperature by running high-density electrical current through the workpiece. Current flow is then removed from the workpiece and an upper die of the die set is closed on the lower die, forming the workpiece into a desired shape. The workpiece may be left between the upper and lower dies of the die set under pressure long enough to cool the workpiece by conductive heat transfer into the upper and lower dies.
Also, U.S. Pat. No. 7,199,334 issued 3 Apr. 2007 to Friedman, et al., discloses a preheating system in which a sheet metal workpiece is placed on a lower platen of a convective heater assembly and sandwiched between an upper platen and the lower platen by actuating the heater assembly to lower the upper platen. The heater assembly then heats the workpiece to a desired temperature by conduction and the upper platen is raised to release the workpiece. The workpiece is then transferred to a forming press by actuating a shuttle assembly. The forming press is then actuated to form the workpiece.
In addition, GM's U.S. Pat. No. 6,890,394 issued 10 May 2005 to Carsley, et al. discloses a method for heating a cold worked sheet of superplastically formable metal composition by placing the sheet between two electrical resistance heated platens that are then closed together to within a critical gap distance of either side of the sheet. The critical gap distance is maintained by positioning shims between the platens before the platens are closed together.
However, an elevated temperature forming method and preheater apparatus constructed or executed according to these patents would be unable to support high volume fabrication of deep drawn panels from sheet metal workpiece of limited formability.
What would be desirable would be an elevated temperature forming system capable of high volume fabrication of deep drawn panels from sheet metal workpieces having insufficient formability at lower temperatures.